Gamepad might be slower, but not to the degree it cripples the player if set up right, even the light shot shouldn't be a problem especially with you having 1-3 seconds time to select the next ability with all the animation lag. Of course if you're used to kb and waiting for the lag to finish, you will be slower on gamepad, because you're starting to navigate to your next skill too late.

The way my bars are set up, i only need to press one key to select my next ability if I, say, mess up a combo on MNK. Of course, there are times I need more seconds to adjust, but it's rarely critical.

About healing, so far I can't remember a case where someone died because of me being too slow on selecting the target, only when I misjudged who will need a heal next (resulting in a midcast death of the target).

tl;dr: gamepad requires you to concentrate more in hectic situations and is a potential handicap, but it doesn't cripple you per default.